The present invention relates in general to power distribution systems, and, more specifically, to integrated harmonic distortion cancellation and transient voltage surge suppression within wall-mounted electrical distribution panelboards and point-of-use devices.
In supplying electricity to a building, AC power from a utility is connected via a service entrance to various types of feeder panels and then on to electrical loads connected to branch circuits. Many of the loads that may be supplied on the branch circuits such as computers, electronic ballasts, variable frequency drives, telecommunications equipment, and laboratory equipment, are non-linear loads. It is known that such non-linear loads create a power quality problem known as harmonic distortion because they draw current at other than the frequency of the input AC voltage. Total harmonic distortion, expressed as THD, is measured as a percentage amount of distortion present on the current or voltage sinewave.
Voltage transients or surges are a major problem affecting sensitive electronics including microprocessor-controlled equipment. Power conditioning devices known as transient voltage surge suppression (TVSS) units or surge protection devices (SPDs) are widely used to reduce this problem. SPDs, however, do not remedy harmonic distortion. As the number of non-linear loads in closely coupled electrical circuits increases, the increasing amount of harmonic distortion can lead to malfunctions of sensitive electronics. The problem of harmonic distortion is discussed in I.E.E.E. Standard 519, “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems”.
Increased levels of harmonic current distortion contribute to adverse thermal effects on electrical equipment. Examples of this problem include premature failure of transformers and nuisance tripping of breakers and fuses. This can even occur on circuits where the actual load current would indicate that the circuit is not overloaded.
Harmonic distortion causes current to flow on the neutral conductor. In a balanced three-phase power system supporting linear loads, very little current is normally present on the neutral conductor. However, power systems that supply electrical power to non-linear loads will always have current flowing on the neutral conductor. The amount of current flowing in the neutral conductor can potentially exceed the amount of current flowing in the phase conductor with the most current load. Since there is no overcurrent device on the neutral conductor this can create a serious problem. In severe cases, harmonic distortion has been found to be the cause of electrical fires in buildings.
Harmonic current distortion also causes distortion of the voltage waveshape. This voltage distortion is measured as a percentage of voltage THD. Excessive voltage THD can cause microprocessor-controlled equipment to malfunction. IEEE Standard 519 states that voltage THD must be kept to less than 5% in order to avoid such malfunctions.
For electrical loads such as variable frequency motor drives, which create significant harmonic distortion, it is known to reduce the distortion flowing back to the distribution system or other loads by coupling a harmonic mitigation device in series with the drive. Standalone components such as K-rated transformers and zig-zag transformers are available for this purpose. For other loads such as the switching DC power supplies in computers and other electronics, the level of harmonic distortion created does not justify the expense of deploying a harmonic mitigating transformer with each load since the distortion from one such load is unlikely to cause noticeable problems even though that distortion propagates back up the branch circuit through its feeder panel and then to loads in other branch circuits. When a greater number of such loads are supplied by a feeder panel, however, the cumulative harmonic current distortion of all the devices can easily cause disruptive overheating problems and if left unchecked will cause voltage harmonic distortion levels to approach and exceed the 5% limit. The TVSS module often attached to or integrated into a feeder panel only protects the loads from transients and is ineffective in controlling the harmonic distortion.